A significant advantage in employing segmented couplings is the ability to quickly assemble pipeline systems on-site. In numerous industrial applications, such as in the gas and oil recovery industries, a particular need arises in the on-site rapid assembly of pipeline systems. It is desirable that such pipeline systems be assembled with maximum economy in labor and materials. Because of the relatively high pressures encountered in such pipeline systems (which for the oil recovery industry commonly are of the order of 4,000 lbs. per square inch or higher) resort has heretofore been made to relatively expensive high-pressure couplings, such as those which require connecting nipples to be welded to the ends of the respective pipes prior to the assembly of the coupling. Such welding operation is undesirable in that the manufacturing operation is performed at a manufacturing plant prior to delivery of the pipes to the assembly site, and represents an increase in cost.
A major problem which has heretofore precluded the use of segmented couplings in high-pressure environments is that of providing adequate inter-engagement of the couplings and the respective pipes, while at the same time maintaining the strength and integrity of the pipes sufficiently high for them to withstand the pressures and forces to which the pipeline system is to be subjected. Segmented couplings require a positive inter-engagement between the coupling segments and the pipes. One typical way of providing this has been by providing a groove adjacent to the pipe end, and, by providing a complementary key on the inner periphery of the coupling segments, such that the coupling segments, when assembled onto the pipe end, are physically keyed into the body of the pipe.
In some industries, where grooving is not possible or desirable, the pipe end is provided with a collar, or with a beaded or raised surface or studs which engage the complementary keys of the coupling.
Such arrangements are admirable in their performance in relatively moderate-pressure applications, but are often times found wanting in high-pressure applications in view of considerations which arise in such high-pressure applications.
While it is possible to form the coupling segments of sufficient strength to withstand the forces exerted on the coupling when in use, problems are encountered in eliminating deformation and fatigue fracture of the pipe itself, which may result in the ultimate total failure of the joint.
These problems arise in part from the desire to employ standard piping which, depending upon the industry and end use, vary from plastic to steel. Such materials when exposed to high pressure are inherently capable of deformation when subjected to stresses at or approaching their elastic limits. While this problem could be alleviated by the use of piping formed from exotic high strength materials, this is not generally economically feasible.
The problems are further aggravated by the necessity of providing a groove or bead, or studding adjacent the end of the pipe for the reception of the key on the coupling. Where grooves are provided, the suggestion has been made to deepen them to provide a greater engagement for the coupling keys, however, this results in a consequential reduction of the available cross-sectional area of the pipe at the location of the groove and the resultant weakening of the pipe at that location. A common point of failure of such joints is between the grooved portion of the pipe and the pipe end, which elastically deforms radially inwards thereby allowing the key to disengage the groove. This produces a wedging or camming action between the groove wall or the bead or the studs and the key which further aggravates the problem. A circumferential collar or bead, or single row of studding adjacent each pipe end similarly provides an insufficient anchor for the coupling when subjected to high pressures.
When grooved pipe is employed, attempts to minimize the depth of the groove, with a consequential increase in the cross-sectional area of the pipe at the location of the groove, result in turn in a corresponding decrease in the surface area of the groove side wall which is available for contact by the key of the coupling. The consequential increase in the stresses between the key and the pipe, again increase the tendency of the material of the pipe to deform at that location under the stresses encountered.